The present invention pertains to improvements in the field of electrocoagulation printing. More particularly, the invention relates to a method of preventing anode abrasion during electrocoagulation printing.
In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, applicant has described a high-speed electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. Where the coloring agent used is a pigment, a dispersing agent is added for uniformly dispersing the pigment into the ink. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is thereafter cleaned by means of a plurality of rotating brushes and a cleaning liquid to remove any residual coagulated colloid adhered to the surface of the positive electrode.
When a polychromic image is desired, the negative and positive electrodes, the positive electrode coating device, ink injector, rubber squeegee and positive electrode cleaning device are arranged to define a printing unit and several printing units each using a coloring agent of different color are disposed in tandem relation to produce several differently colored images of coagulated colloid which are transferred at respective transfer stations onto the substrate in superimposed relation to provide the desired polychromic image. Alternatively, the printing units can be arranged around a single roller adapted to bring the substrate into contact with the dots of colored, coagulated colloid produced by each printing unit, and the substrate which is in the form of a continuous web is partially wrapped around the roller and passed through the respective transfer stations for being imprinted with the differently colored images in superimposed relation.
Applicant has observed that the metal oxide used in combination with the olefin substance for coating the positive electrode causes abrasion and pitting of the positive electrode so that it is necessary to regrind the surface of such an electrode after every forty hours of printing. This of course requires shutdown of the printing apparatus and removal of the electrode. Where a positive electrode made of stainless steel or aluminum is utilized, Fe.sup.3+ or Al.sup.3+ ions are released from the surface of the electrode as a result of the abrasion and pitting thereof. As explained in Applicant's copending U.S. application Ser. No. 08/376,245, filed on Jan. 23, 1995 these ions crosslink the colloid contained in the ink, resulting in a viscosity increase leading to an ultimate gelation of the ink.
Applicant has also observed that the metal oxide in combination with the olefinic substance retain on the surface of the positive electrode a film of ink which is transferred with the colored, coagulated colloid onto the substrate during contact with same. Thus, when black, cyan, magenta and yellow coloring agents are used to provide a polychromic image, the residual films containing these coloring agents upon being transferred onto the substrate in superimposed relation create on the printed image an undesirable colored background.